Wording fix.

Doc/library/concurrent.futures.rst

@@ -221,7 +221,7 @@ The :class:`Future` class encapsulates the asynchronous execution of a callable.
     .. method:: cancel()
 
        Attempt to cancel the call.  If the call is currently being executed and
-       cannot be cancelled and the method will return ``False``, otherwise the
+       cannot be cancelled then the method will return ``False``, otherwise the
        call will be cancelled and the method will return ``True``.
 
     .. method:: cancelled()